Lactic Acidosis
Lactic acidosis is a digestive disorder in ruminants that may occur when there is a sudden excess intake of readily fermentable carbohydrates, particularly when ruminants are switched from a diet of roughage to a high-energy or energy rich concentrate diet containing a high level of starch. The disorder is characterised by an accumulation of organic acids, especially lactic acid, in the rumen (Dawson & Allison, 1988). Studies have indicated that a gross imbalance between the numbers of lactic acid-producing bacteria and lactic acid-utilising bacteria, brought on by a sudden increase in the proportion of readily fermentable carbohydrates in the diet is the main cause of the onset of lactic acidosis (Slyter, 1976).
Manipulating the rumen microbial population to prevent lactic acidosis by administering material containing high numbers of lactate-utilising bacteria has been advocated for decades, but never practiced on a large scale. Manipulations to enhance lactate utilisation within the rumen has been achieved by administering rumen fluid from an already adapted animal (Allison et al., 1964; Braun et al., 1992) and by administering pure or mixed bacterial cultures of lactate-utilisers (U.S. Pat. No. 1,251,483 Wilker et al., 1971; U.S. Pat. No. 3,857,971 Abdo & Cahilly, 1974; U.S. Pat. No. 4,138,498 Das, 1979; U.S. Pat. No. 5,380,525 Leedle et al., 1991; Hession & Kung, 1992; Robinson et al., 1992; Wiryawan & Brooker, 1995).
Some of these feed additives containing live bacterial cultures have been patented (U.S. Pat. No. 1,251,483 Wilker et al., 1971; U.S. Pat. No. 3,857,971 Abdo & Cahilly, 1974; U.S. Pat. No. 4,138,498 Das, 1979; U.S. Pat. No. 5,380,525 Leedle et al., 1991), but not commercialised extensively or at all. In three of the patents (U.S. Pat. No. 1,251,483 Wilker et al., 1971; U.S. Pat. No. 3,857,971 Abdo & Cahilly, 1974; U.S. Pat. No. 4,138,498 Das, 1979) the cultures were obtained from continuous culture fermenters with an initial inoculum of rumen fluid. However, the donor animals were not necessarily adapted to a high-concentrate diet. There is also no mention of pH tolerance for any of these cultures. In the other patent (U.S. Pat. No. 5,380,525 Leedle et al., 1991) the cultures were isolated at pH 5.3 either directly or indirectly after enrichment from ruminants adapted to high-concentrate diets.
The Incidence of Sub-Acute and Acute Acidosis in Dairy Cattle
Sub-acute rumen acidosis is a common and serious health and production problem in the dairy industry because diary cows are usually fed diets containing high levels of grains. Sub-acute and acute rumen acidosis are simply different degrees of the same problem. Acute rumen acidosis is more severe and physiological functions may be significantly impaired. The affected animal is depressed and usually ataxic, off-feed, with dilated pupils and an elevated heart rate. Diarrhoea will be obvious and the animal may become recumbent and die within 2 to 5 days after the insult (Nordlund, 1995). Acute acidosis is characterised by a dramatic reduction in ruminal pH (≦5.0), a large increase in lactic acid concentration and a large decrease in protozoa (Nocek, 1997).
Signs of sub-acute rumen acidosis are very different from that of acute acidosis. Modern dairy management systems of group housing or group feeding make it difficult to recognise these symptoms because individual cows with these problems will usually not be noticed within a group. Herds with sub-acute rumen acidosis will present some or all of the following signs: laminitis, intermittent diarrhoea, poor appetite or cyclical feed intake, high herd cull rates for poorly defined health problems, poor body condition in spite of adequate energy intake, abscesses without obvious causes and hemoptysis (coughing of blood) or epistaxis (bleeding from the nose). Most of these signs are secondary to acidosis and most of them do not appear until weeks or months after the initial acidosis events. Contrary to feedlot cattle, dairy cows are kept for years and the management of acidosis is therefore of importance in increasing profits.
Chronic laminitis is perhaps the most consistent clinical sign of a herd with sub-acute rumen acidosis. Although the relationship between acidosis and laminitis is not completely understood, the association is widely recognised clinically and demonstrated in research trials (Kelly & Leaver, 1990; Manson & Leaver, 1988; Nocek, 1997). Furthermore, most dairy managers, veterinarians and nutritionists tend to underestimate or perhaps tolerate an abnormal incidence of laminitis and lameness in dairy herds. A survey in Minnesota demonstrated a mean incidence in lameness of 15% with a range of 0-33% (Nordlund, Garret & Oetzel, 1995). Studies in Europe have identified lameness as the third most costly health problem in dairy cows after mastitis and reproduction (McDaniel & Wilk, 1989). The management of acidosis is thus clearly of utmost importance.
A major symptom of sub-acute acidosis is decreased feed intake and decreased efficiency of milk production. Sub-acute acidosis, because of difficulties in diagnosing the problem, tends to be dismissed as other problems, such as poor management, poor forage quality e
biggest economic sink to many dairy farmers because it is omni-present, particularly in high producing dairy herds.
Because of the high incidence of nutritional and metabolic disturbance amongst high producing dairy cows, nutritional strategies for improving performance with cereal based diets focus on the prevention of ruminal dysfunction by controlling acid production or by stimulating more efficient microbial growth. At present, feed additives play an important role in this regard (Hutjens, 1999). The use of yeast culture strains that specifically stimulate the growth of lactic acid utilising bacteria generates much interest and a recent survey indicated that yeast cultures are being used in 33% of high producing Wisconsin herds. Results from various studies suggest that the Yea Sacc strain 84170 appears to be particularly well suited for altering ruminal fermentation and animal production when used in high lactate silages and feeds high in concentrates (Dawson, 1995). Production results, however, are very inconsistent. In the USA the cost for yeast culture supplementation is 4-6 cents per cow per day (Hutjens, 1999). Ionophores, because of their ability to prevent the growth of important lactic acid producers, can also play a role in managing sub-acute acidosis. Although the cost is relatively low (1-2 US cents/cow/day, Hutjens, 1999) there seems to be some resistance against the use of ionophores because of a few recent cases of ionophore toxicity. Furthermore, ionophores have not been registered in the USA for use in dairy cattle diets.
Experimentally, there have been several bacteria that have potential as direct fed microbials (DFM) for ruminants, but have not been commercialised for a number of reasons. For example, Megasphaera elsdenii (ME) is the major lactate-utilising organisms in the rumen of adapted cattle fed high grain diets. When cattle are shifted from high forage to high concentrate diet, the numbers of ME are often insufficient to prevent lactic acidosis. Kung and Hessian (1995) have shown that the addition of ME B 159 prevented accumulation of lactic acid during a challenge with highly fermentable carbohydrates. Robinson et al (1992) demonstrated that addition of a different strain of ME (407A) prevented lactic acidosis in steers.
Although the costs associated with subclinical ruminal acidosis are difficult to pinpoint, the potential costs to the dairy industry are huge (Hall, 1999). Donovan (1997) conservatively estimated the cost of subclinical acidosis to the US dairy industry at $500 million to $1 billion per year.
Elsden and Lewis (1953) first described a large, strictly anaerobic Gram-negative, fatty acid producing, non-motile coccus isolated from the rumen of sheep. However, the original isolate was lost before it had been characterised phenotypically in detail. An organism resembling the original strain was isolated from the rumen contents of sheep several years later by Elsden and his colleagues (Elsden et al., 1956). The characteristics of this organism did not fit the description of any known species at the time, but in view of the small number of isolates studied, the authors refrained from assigning the organism to a new species and genus, but referred to it as LC. Gutierrez et al. (1959) encountered a similar organism in the rumens of bloating cattle and concluded that they fell within the definition of the genus Peptostreptococcus, proposing the creation of a new species P. elsdenii. Subsequently, Rogosa (1971) demonstrated that the LC-type isolates were Gram-negative and therefore should not be included in the genus Peptostreptococcus. He proposed transfer of P. elsdenii to a new genus Megasphaera and the new combination M. elsdenii, with the isolate LC1 of Elsden et al. (1956) as the type strain. M. elsdenii is a strict anaerobe found mainly in the rumen of young animals and animals receiving high-concentrate diets in which lactate fermentation is particularly pronounced. The organism has also been isolated on occasion from the faeces of humans (Sugihara et al., 1974) and it ferments lactate to mainly butyrate, propionate, isobutyrate, valerate, CO2, H2 and sometimes trace amounts of caproate (Stewart and Bryant, 1988). Since M. elsdenii is not subject to catabolite repression by glucose or maltose as in Selenomonas, which is also a lactate utiliser occurring in the rumen, its contribution to lactate catabolism is particularly enhanced subsequent to feeding of soluble carbohydrates (Stewart and Bryant, 1988).
U.S. Pat. No. 3,956,482 (Hahn et al 1976) discloses a method of increasing milk production in ruminants including the steps of administering to the rumen of a lactating cow acetate producing micro-organisms consisting of a mixture of 0-4% M. elsdenii, 30-42% Streptococcus bovis, 3-10% Lactobacillus acidophilus, 12-20% Bifidobacterium adolescents, 18-44% Bacteroides ruminicola and 3-12% Butytrivibrio fibrisolvens cultured and adapted to a nutrient medium.
A major disadvantage of the invention disclosed in the above patent is the relatively high percentage (between 30-42%) of Streptococcus bovis, which together with Lactobacillis is the leading cause of lactic acidosis in ruminants. The mixture further contains a relatively low percentage of M. elsdenii (0-4%) and the administration of the mixture would probably aggravate or initiate ruminal lactic acidosis rather than preventing or treating it. The mixture is further exposed to atmosphere so that most of the M. elsdenii perish. A mixture of microorganisms is furthermore much more difficult to control than a pure culture.
U.S. Pat. No. 4,138,498 (Das, 1979) discloses a feed additive for administration to ruminants to prevent or minimise lactic acidosis when ruminants are switched from a diet of roughage to starch, comprising a bacterial culture of M. elsdenii admixed with an ingestible animal feed additive. M. elsdenii is strictly anaerobic and a disadvantage of the feed additive disclosed in this patent, over and above the disadvantages set out below, is that the M. elsdenii is exposed to atmosphere, leading to a rapid decline in the amount of viable cells available in the additive.
U.S. Pat. No. 5,380,525 (Leedle et al., 1991) discloses a biologically pure culture of M. elsdenii NRRL-18624 and its use in the facilitation of the adaptation of ruminants from a roughage or normal pasture to a high-energy starch-rich diet. The culture suffers from the disadvantages set out below.
U.S. Pat. No. 5,529,793 (Garner et al., 1996) discloses a mixture of lactic acid producing bacteria and a lactate utilising bacteria such a M. elsdenii with a dry formulation or an animal feedlot diet for improving the utilisation of feedstuffs by a ruminant. A disadvantage of this invention is that M. elsdenii is generally strictly anaerobic and the application thereof to dry feedstuffs would result in most of the cells dying.
The applicants have evaluated the above strains of M. elsdenii and have deducted that they are generally not suitable for commercialisation and large scale preventative treatment of lactic acidosis in ruminants because of the following disadvantages of these strains namely they are not:                highly active and adapted to proliferate in the rumen of animals on high-concentrate diets;        capable of proliferating at relatively low pH values below pH 5.0 and as low as 4.5, characterised as acute acidosis;        resistant to ionophore antibiotics commonly added to feedlot diets; and        capable of preferentially using lactate as a carbon source even in the presence of soluble carbohydrates such as glucose and maltose.        
Further disadvantages of these strains are that, generally, they:                have a relatively low growth rate, i.e. less than 0.938 h−1;        do not have the ability to grow on reducing sugars as well as on lactate;        have a relatively low biomass output rate, i.e. less than 0.39 g (l.h)−1;        are not ionophore resistant; and        produce predominantly propionate and butyrate and not predominantly acetate.        